The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus, and especially to an image forming apparatus by which a multi-color image can be obtained by transferring toner images onto a transfer sheet after toner images have been superimposed on an image carrier.
Conventionally, for example, the following method has been proposed and practiced in order to form multi-color images by an electrophotographic method: image forming processes of charging, exposing, and developing are repeated at each component color; each color toner image is superimposed onto an image carrier; and after that, the multi-color toner image is transferred onto a transfer sheet.
The principle of the image forming method will be explained as follows referring to a flow chart in FIG. 5. FIG. 5 shows changes of the surface potential of a photoreceptor which is an image carrier, and the charged polarity of which is negative.
The photoreceptor is uniformly charged by a scorotron charger, and given a predetermined negative surface potential V.sub.H. The surface potential V.sub.H is lowered to V.sub.L, close to a zero potential, by a first image exposure using an exposure source such as a laser, a cathode ray tube, a liquid crystal shutter, or an LED.
When the image is developed by a developing means upon which a negative bias voltage, a DC component of which is almost equal to the surface potential V.sub.H of an unexposed portion, is impressed, negatively charged toner T in the developing means sticks to a relatively low potential exposed portion. In this case, the lower the potential is, the larger the stuck amount is. Due to the foregoing, a first color visual image is formed.
Potential of the area on which the visual image has been formed is increased when negatively charged toner T has been stuck onto the area. Next, when second charging is performed, the potential is increased, and the same initial surface potential V.sub.H as that of the unexposed portion is obtained.
Next, a second image exposure is performed on the surface of the photoreceptor on which the uniform surface potential V.sub.H has been obtained, and an electrostatic latent image is formed thereon. Then, the second color visual image is obtained after the same developing operation as the foregoing.
When the above-described processes are repeated, the multi-color toner image can be obtained on the photoreceptor. Only the image portion, which is developed with toner T, is re-exposed so that inferior transferring such as transfer repelling does not occur and excellent transferring can be performed, even when voltage resistance of the transfer sheet is lowered due to high humidity. After that, the multi-color toner image is transferred onto the transfer sheet by a transfer means, and further heated or pressurized to be fixed, so that the multi-color image can be obtained.
In the image forming apparatus by which a multi-color image is obtained according to the foregoing method, a plurality of developing devices are provided around the photoreceptor so that toner images are superimposed on the photoreceptor. Therefore, the diameter of the photoreceptor drum becomes large, and separability of the transfer sheet is deteriorated. Accordingly, sufficient separability can not be obtained by the conventional electrostatic transfer separation type of apparatus. Due to the foregoing, a more positive separation means is necessary. In apparatuses of the type in which the toner image is superimposed, an amount of stuck toner is increased, and a large transfer charge amount is necessary. Accordingly, a transfer belt apparatus is used which has a large transfer charge holding property and excellent separability.
In the above-described multi-color image forming method, a reversal developing method is used as a developing method for an electrostatic latent image. In the reversal developing method, only a toner image forming portion on the photoreceptor is exposed, and since it is not necessary to expose all the background portion as in the case of regular development, the load onto the photoreceptor can be decreased.
Further, by the charging operation for the second time or after, the potential of the visual image which has been formed before charging becomes V.sub.H, and the image is not developed if the image is not written again, so that mixing color deteriorations do not occur, which is an advantage of the reversal developing method.
In the image forming apparatus for multi-color images, when a transfer belt apparatus and a cleaning apparatus, are pressed onto the photoreceptor drum during the above-described re-exposure, traveling speed of the photoreceptor drum is drastically changed because of the load onto the photoreceptor. Due to this, a re-exposure position is shifted as shown in FIG. 6(b). Therefore, there are problems that: a low potential portion is generated near the toner image; toner T is moved as shown in FIG. 6(c); and resolution is lowered and color offset is caused due to splashing of toner T. Further, there are problems that the potential of the finally formed toner image portion is excessively lowered when the re-exposure is performed, and therefore, the transfer ratio is lowered. The first object of the present invention is to provide an image forming apparatus in which re-exposure is performed at a correct position, and resolution is not lowered and color offset is not caused. Further, the second object of the present invention is to provide an image forming apparatus in which toner is not splashed, and a high transfer ratio can be obtained.